1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for manufacturing the same.
2. Description of Related Art
Nowadays, CCD (Charge Coupled Device) solid-state imaging devices using a CCD for reading out a signal charge are in the mainstream. Also, in solid-state imaging devices, as pixels become finer and finer, there is a considerable increase in the number of pixels and a significant reduction in the size of an imaging element.
In general, a solid-state imaging device used in a video camera, a digital still camera, or the like includes a photodetector portion (photodiode), a vertical transfer CCD portion (vertical CCD), and a horizontal transfer CCD portion (horizontal CCD), which are formed on a silicon substrate. The photodetector portion performs photoelectric conversion, and accumulates a signal charge in accordance with received light. The vertical transfer CCD portion reads out the signal charge accumulated in the photodetector portion, and transfers the same in a vertical direction. The horizontal transfer CCD portion (horizontal CCD) transfers the signal charge transferred by the vertical transfer CCD portion in a horizontal direction.
On the silicon substrate, a transfer gate electrode constituting the vertical transfer CCD portion or the horizontal transfer CCD portion is formed via an insulating film. Further, on the silicon substrate, there are an interlayer insulating film, a light-shielding film having an opening above the photodetector portion, and a surface protective film in this order. Further, a flattening film, a color filter, and a microlens are layered in this order, as necessary.
There are a plurality of the photodetector portions, which are arranged in a matrix in horizontal and vertical directions. The vertical transfer CCD portion is provided with respect to each vertical row of the photodetector portions so as to be in parallel with the row. In the case of interline transfer, the vertical row of the photodetector portions and the vertical transfer CCD portion are arranged alternately. One photodetector portion and a part of the vertical transfer CCD portion corresponding thereto that is adjacent to the photodetector portion constitutes a single pixel.
In this configuration, when a predetermined signal voltage is applied to the transfer gate electrode so as to drive the respective CCD portions, a signal change generated by the incidence of light on the photodetector portion is transferred to the vertical transfer CCD portion and the horizontal transfer CCD portion sequentially. The signal charge is output finally as an image signal from an output circuit connected with the horizontal CCD.
In some solid-state imaging devices, an output signal may be observed even when incident light is shielded completely. This is a kind of noise signals referred to as dark current or white flaws. Such a noise signal is known to increase exponentially with temperature.
At present, in general, a buried photodiode is used as a structure of the photodetector portion so as to suppress the generation of dark current and white flaws. A buried photodiode is configured as follows: a semiconductor region (surface inversion layer) having an inverse conductivity type is provided on a photoelectric conversion region (semiconductor region) formed on a silicon substrate.
The buried photodiode suppresses dark current and white flaws caused by an interface state between the photodiode and a surface oxide film (insulating film) or crystal defects formed in the vicinity of a surface of the photodiode by the surface inversion layer. The buried photodiode is manufactured in the following manner, for example. When the photodiode has an n-type surface (photoelectric conversion region), a p-type impurity (boron (B)) is ion-implanted in the surface to a shallow depth so as to form a p-type surface inversion layer. After the ion implantation, a heat treatment such as annealing is performed so as to repair crystal defects formed on the silicon substrate by the ion implantation. The buried photodiode manufactured in this manner allows a thermally excited electron to recombine with a hole formed of the p-type impurity, thereby reducing dark current and white flaws.
However, in the buried photodiode, when the impurity concentration on the uppermost surface is reduced due to variations in the concentration of the impurity during ion implantation, there is a decrease in the ability of the surface inversion layer to suppress dark current and white flaws. Further, when the dose of ion implantation for the surface inversion layer is increased so as to suppress a reduction in the impurity concentration on the uppermost surface, crystal defects on the silicon substrate are increased by the ion implantation when the dose of ion implantation exceeds an optimum level. As a result, the number of white flaws is increased again. Further, even when the dose of ion implantation for the surface inversion layer is optimum, the generation of white flaws cannot be reduced to a certain level or lower.
In order to solve the above-mentioned problems, JP 6(1994)-163971 A proposes a manufacturing method in which in forming a surface inversion layer, a layer having the same conductivity type as that of a conventional surface inversion layer and a higher impurity concentration than that of the conventional surface inversion layer is formed on a surface side of the conventional surface inversion layer. With the manufacturing method described in the above document, it is considered that a reduction in the impurity concentration on the uppermost surface due to variations of the impurity during ion implantation can be suppressed. Further, it is possible to suppress the formation of crystal defects on the silicon substrate during ion implantation, and accordingly an increase of crystal defects due to an increased dose of ion implantation can be suppressed. Further, it is considered that the generation of white flaws when the dose of ion implantation is optimum can be reduced further.
However, boron (B), which is ion-implanted for forming the surface inversion layer, is likely to be diffused by heat. Accordingly, when boron (B) is introduced during the formation of the surface inversion layer, the boron (B) is diffused toward the photoelectric conversion region by a heat treatment. Consequently, it is difficult to distribute the p-type impurity on the surface of the photodiode to a shallow depth in a concentrated manner. Moreover, the diffusion of boron (B) narrows the photoelectric conversion region.
For these reasons, according to conventional manufacturing methods including the one described in the above document, there is a small amount of donor produced by the photodiode, and it is impossible to increase the saturation electric charge (maximum storage electric charge).
Further, even with the manufacturing method described in the above document, it is impossible to suppress sufficiently an increase of crystal defects on the silicon substrate due to an increased dose of ion implantation. Further, it is also impossible to reduce sufficiently the generation of white flaws when the dose of ion implantation is optimum. Thus, it is required to suppress further the formation of crystal defects on the silicon substrate during ion implantation and to reduce further the generation of white flaws.
It is an object of the present invention to provide a solid-state imaging device that solves the above-mentioned problems, reduces the generation of dark current and white flaws as compared with conventional examples, and increases the saturation electric charge of a photodiode, and a method for manufacturing the same.